1. Field of the Invention
The present invention relates to a screen display for displaying characters and the like on a predetermined range of the screen of a display such as a CRT and a liquid crystal display, and more particularly, to a control system for a screen display which enables vertical or horizontal scroll display of display contents from one side of the screen to the other.
2. Description of the Prior Art
Conventionally, images received by commercial radio waves are displayed on the screen of a CRT, and characters and patterns indicating channel number and various operation conditions are displayed over the received images. FIG. 14 is a block diagram of the conventional screen display. Reference numeral 1 represents a read address control circuit, 5 a synchronizing signal input circuit, 6 an oscillator circuit, 7 an H counter, 8 a timing generator, 9 a display position detection circuit, 10 an input control circuit, 11 an address control circuit, 12 a display control register, 13 a display data RAM, 14 a data control circuit, 15 a character ROM, 16 a synthesizer circuit, 17 a shift register and 18 a display control circuit.
The operation of the display will be described hereafter.
When the synchronizing signal input circuit 5 receives a horizontal synchronizing signal HSYNC and a vertical synchronizing signal VSYNC, it inputs synchronizing signals based on these synchronizing signals into the oscillator circuit 6 and the H counter 7. The oscillator circuit 6 inputs a predetermined oscillation output based on the input synchronizing signal into the timing generator 8, which generates a timing signal required for the operation of each part based on the oscillation output for output to each part. The H counter 7 is reset by each vertical synchronizing signal and counts the horizontal synchronizing signal. A count value of the H counter is provided to the display position detection circuit 9 for detecting the display position of characters and patterns. The display position detection circuit 9 detects an image display position on the screen which is set in advance according to the input synchronizing signals and adjusts timing to display the received image.
Meanwhile, an image data CD provided from an unshown microcomputer for displaying an image is inputted into the input control circuit 10. The address control circuit 11 specifies addresses in the display control register 12 and the display data RAM 13 according to an input address. The display control register 12 and the display data RAM 13 have different addresses on the same address space, whereby the above-mentioned input data is written onto the specified addresses in the display control register 12 and the display data RAM 13 through the data control circuit 14. The data include character code, color information, display mode and display position data.
The display position detection circuit 9 compares display position data stored in the display control register 12 and a count value of the H counter 7. When they coincide, the circuit provides a coincidence signal to the read address control circuit 1. The read address control circuit 1 is thereby activated to start reading the written data by specifying an address in the display data RAM 13. The display data RAM 13 provides an address for a written character code to the character ROM 15, by which a character font corresponding to the code is read from the character ROM 15.
A single font consists of l.times.m-dot pixels, for example, as shown in FIG. 15. Therefore, the character ROM 11 has a memory capacity of l.times.m.times.n dots for storing fonts for n characters. Font data read from the character ROM 15 are produced by the synthesizer circuit 16 when necessary. The output data of the synthesizer circuit 16 are converted from parallel format to serial format by the shift register 17 and provided to the display control circuit 18. The display control circuit 18 receives color information data including color and background color from the display data RAM 13 and display mode data indicating the ornamentation of characters from the display control register 12 in addition to font data from the shift register 17 for display control of the font data and color information data according to a display mode specified by the display mode data. Thus, red, green and blue output signals and a luminance control signal are generated from the display control circuit 18, and desired characters and patterns are displayed on the screen according to these signals.
Every dot of font dot data in vertical direction is stored in the address area of the character ROM 15 in which the font dot data are produced by decoding character code data representing each font. In the case of FIG. 15, vertical one-dot font data is stored in each line having an address which is called AD1, AD2 . . ., AD1-1, and AD1 from the top. These dot data are outputted from the character ROM 15 from the top. The read address control circuit 1 counts from one to l for the vertical lines of a single character, and specifies an address in the display data RAM 13 when it completes the display of an l-th line. Thereafter, the dot data of the character ROM 15 corresponding to a character code is outputted one after another from the display data RAM 13.
Vertical scroll display using the above-mentioned screen display will be described hereafter. Vertical scroll display means that display contents move from the bottom (or the top) to the top (or the bottom) of the screen.
Each time an interrupt is generated for each vertical synchronizing signal (VSYNC), the vertical position data of the display control register 12 is rewritten by the unshown microcomputer. At this time, when the display position data is decremented for each vertical synchronizing signal, displayed characters look as if they are scrolled from the bottom to the top of the screen (refer to FIG. 16). When the display position data is incremented for each vertical synchronizing signal, displayed characters look as if they are scrolled from the top to the bottom of the screen (unshown). In this way, as shown in FIG. 16, the screen S of a display such as a CRT or a liquid crystal display is scanned by a scanning line F from the top Sa to the bottom Sb to display display contents N such as characters and figures on the screen S according to the output of the screen display while displaying an image received by commercial radio waves. The display contents N can be scrolled from the bottom to the top of the screen or vice versa over a scroll section P having a predetermined width. In other words, the display contents N in the scroll section P are formed by scanning the scanning line F from the display start position Pa, the top side of the scroll section P on the opposite side of a scroll direction, to the display end position Pb, the bottom side of the scroll section P.
Display contents can be scrolled from left to right, or vice versa by setting the scroll section P in the horizontal direction of the screen.
Since the conventional screen display is structured as described above, vertical scroll display is such as shown in FIG. 16. Therefore, to display "C" below the line for "B", the entire font for "C" is always displayed, and natural scroll display in which a character font on the lowest line is displayed little by little from a predetermined position Pt as shown in FIG. 17 is impossible. The same can be said of scroll display from the top to the bottom of the screen.